Iron control as part of a well treatment using time-released agents

ABSTRACT

In a well in a subterranean formation, treatment fluids for time-released iron control are utilized. These treatment fluids contain a time-released form of a complexing agent or reducing agent, wherein the agent forms a soluble product with iron. The time-released form prevents loss of the agent prior to delivery to the desired site and at the desired time(s). Methods for controlling iron in a well in a subterranean formation utilize these treatment fluids containing a time-released form of a complexing agent or reducing agent and these methods create soluble products when the agent interacts with the iron to control the iron from the formation and/or well.

RELATED APPLICATION

The present application is a Continuation of U.S. Pat. Application No.17/170,290 filed Feb. 8, 2021, which claims benefit of U.S. ProvisionalNo. 62/971,451 filed Feb. 7, 2020, the contents of both of which areherein incorporated by reference in their entireties.

INTRODUCTION

Water-based treatment fluids are used in hydrocarbon recovery in oil andgas wells. In particular, water-based treatment fluids are used infracturing of oil and gas wells. The well contains a well casing and, insome wells, tubing inside the casing. Perforations or ports in thecasing are adjacent to targeted intervals of subterranean formationscontaining a hydrocarbon.

Fracturing is conducted by pumping at high pressures and high velocitiesthrough a vertical and, in some instances, a horizontal section of awell. Hydraulic pressure exerted on the formation, above the fracturinggradient, causes the formation to fracture, creating an extensivefracture network. These formations can have low or minimal permeability,including sandstone, shale or coals. Once the fracture or crack isinitiated, pumping is continued, allowing the fracture to propagate.

Once the fracture has gained sufficient fracture width, a proppant suchas sand is added to the fluid and is transported to the fracture system,partially filling the fracture network. After the desired amount ofproppant is placed in the fracture, additional water-based fluid ispumped to flush the casing of any proppant that may have settled in thecasing. On completion of the fracturing process, the fracture closesonto the proppant. When the well is opened, as the pressure is relieved,a portion of the treatment fluid is flowed back from the well, creatinga conductive pathway needed to accelerate oil and gas recovery from theformation.

Water-based treatment fluids are used both in the initial fracturingprocess and in remediation processes to prolong the useful life of theoil and gas well for producing hydrocarbons.

Iron control presents a significant and complex problem in welloperations, both in the initial fracturing process and in remediationprocesses. Iron and iron compounds may originate from the well equipmentor formation.

During the fracturing process, proppant causes erosion of the inside ofmetal pipe. Pipe metal pumped downhole can serve as a prolonged sourceof iron.

The water-based treatment fluid may contain an acid, such as HC1, or anoxidizer, such as CIO₂, that dissolves, solubilizes, and mobilizes ironfrom multiple sources. Sources of iron can include, for example, rust instorage and mixing tanks, iron corrosion products including acidcorrosion in well tubulars (i.e., pipe), iron scale (iron sulfide andiron hydroxide), source waters, and iron-containing minerals foundnaturally in the formation. If iron is not controlled, it may form iron(III) hydroxide that causes plugging.

Thus, it is desirable to control iron in a well in a subterraneanformation.

SUMMARY

This disclosure describes embodiments of methods and time-releasecompositions that control iron in a well in a subterranean formation.

In one aspect, this disclosure describes a treatment fluid fortime-released iron control in a well in a subterranean formationcomprising a time-released form of a complexing agent or reducing agent,wherein the agent forms a soluble product with iron. In certain aspectsthe time-released form of the complexing or reducing agent is in a formthat provides a varying time-release profile for the agent. Thesevarying time-release profile may provide for peaks of release of theagent (two, three, or four peaks of release) or may provide for acontinuous timed release profile from a start time to an end time.

In another aspect, this disclosure describes a method for controllingiron in a well in a subterranean formation. The method comprises thesteps of providing a treatment fluid comprising a time-released form ofa complexing agent or reducing agent, wherein the agent forms a solubleproduct with iron; injecting the treatment fluid into the well of theformation until at least some of the treatment fluid contacts theformation and iron; and maintaining the treatment fluid in contact withiron within the formation, thereby allowing the complexing agent orreducing agent to release and interact with iron to form a solubleproduct. In certain embodiments of the methods described herein, thetime-released form of the complexing or reducing agent is in a form thatprovides a varying time-release profile for the agent. These varyingtime-release profile may provide for peaks of release of the agent (two,three, or four peaks of release) or may provide for a continuous timedrelease profile from a start time to an end time.

In these compositions and methods the time-released agent may be acomplexing or reducing agent that forms a soluble product by interactingwith iron. Also in these compositions and methods, the time-release maybe achieved by any form, type or delivery system that prevents loss ofthe agent prior to delivery within the well or the formation where theiron control is needed and at the desired time(s). In certainembodiments, the time-released form of the complexing or reducing agentis in a form that provides a varying time-release profile for the agent.

These and various other features as well as advantages whichcharacterize the systems and methods described herein will be apparentfrom a reading of the following detailed description and a review of theassociated drawings. Additional features are set forth in thedescription which follows, and in part will be apparent from thedescription, or may be learned by practice of the technology. Thebenefits and features of the technology will be realized and attained bythe structure particularly pointed out in the written description andclaims hereof as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWING

The following Figures, which form a part of this application, areillustrative of described technology and are not meant to limit thescope of the disclosure as claimed in any manner.

FIG. 1 represents an embodiment of a method controlling iron in a wellin a subterranean formation using a treatment fluid comprising atime-released form of a metal complexing agent or reducing agent,wherein the agent forms a soluble product with iron.

FIG. 2 represents an embodiment of a method controlling iron in a wellin a subterranean formation using a treatment fluid comprising atime-released form of a metal complexing agent or reducing agent,wherein the agent forms a soluble product with iron. In this embodiment,the agent is released with a varying time-release profile.

FIG. 3 graphically represents a varying time-released profile with twopeaks of release of agent. As illustrated, the injection time is t₀. Afirst release time is at t₁ and a second time-release profile is at t₂.In certain embodiments, t₁ may be about 10 minutes after injection andt₂ may be as late as about 1 month after injection.

FIG. 4 graphically represents a varying time-released profile with threepeaks of release of agent. As illustrated, the injection time is t₀. Afirst release time is at t₁, and a second time-release profile is at t₂,and a third time-release profile is at t₃. In certain embodiments, t₁may be about 10 minutes after injection, t₂ may be a time more thanabout 10 minutes after injection and less than about 1 month afterinjection, and t₃ may be as late as about 1 month after injection.

FIG. 5 graphically represents a varying time-released profile with acontinuous timed release profile. As illustrated, the injection time ist₀. The continuous release begins at a time t₁ and ends at a time t₂. Incertain embodiments, t₁ may be about 10 minutes after injection and t₂may be as late as about 1 month after injection.

DETAILED DESCRIPTION

Before the methods and time-release compositions that control iron in awell in a subterranean formation are disclosed and described, it is tobe understood that this disclosure is not limited to the particularstructures, process steps, or materials disclosed herein, but isextended to equivalents thereof as would be recognized by thoseordinarily skilled in the relevant arts. It should also be understoodthat terminology employed herein is used for the purpose of describingparticular embodiments only and is not intended to be limiting. It mustbe noted that, as used in this specification, the singular forms “a,”“an,” and “the” include plural referents unless the context clearlydictates otherwise. Thus, for example, reference to “a complexing agent”is not to be taken as quantitatively or source limiting; reference to “astep” may include multiple steps, reference to “producing” or a“complex” should not be taken to be all of the products of a reaction,and reference to “reacting” may include reference to one or more of suchreaction steps. As such, the step of complexing can include multiple orrepeated reaction of similar materials to produce identified complexedproducts.

This disclosure describes embodiments of methods and time-releasecompositions that can be used to control iron within a well and/or theformation, both naturally occurring and introduced iron, therebyimproving performance of the well and/or the fracturing operation.

In fracturing of hydrocarbon-bearing formations, including shales and inremediation processes, it has been observed that iron needs to becontrolled. Iron and iron compounds may originate from the wellequipment or from the formation, for example, rust in storage and mixingtanks, iron corrosion products including acid corrosion in well tubulars(i.e., pipe), iron scale (iron sulfide and iron hydroxide), sourcewaters, and iron-containing minerals in formation. Ferrous iron (Fe²⁺)may be present and may be oxidized to ferric iron (Fe³⁺). Ferric ironmay form undesirable, insoluble products within the well and/orformation. The present methods and time-release compositions interactwith these insoluble iron products and form soluble iron products tocontrol the iron from the well and/or formation. As such, the presentmethods and time-release compositions control and interact with ferrousiron (Fe²⁺) and ferric iron (Fe³⁺).

Without being held to a particular theory, it appears that the ironoriginating from pipe erosion can provide a prolonged source of iron.Time-released iron control is advantageous because the iron controlagents otherwise may interfere with chemicals used in the fracturingtreatment. As such, the methods and compositions disclosed herein avoidthis interference by releasing the agent at a desired time(s).

Additionally, without being held to a particular theory, it appears thatthe iron, which is solubilized under conditions of the well treatments,may later form undesirable products, such as iron (III) hydroxideprecipitation. The compositions and methods as disclosed herein form asoluble iron product that is less harmful and easier to address, andthus more desirable, than iron (III) hydroxide. Also, the time-releasedagents allow the agents to be delivered to the site of action andreleased at a desired time(s). For example, some iron control chemicals(e.g., EDTA) have low solubility in acid treatment fluids and throughtime-release, these chemicals can be delivered to the site of actionefficiently and effectively.

This disclosure describes embodiments of methods and compositions thatcan be used to control iron within a well and/or the formation, bothnaturally occurring and introduced iron, thereby improving performanceof the well and/or the fracturing operation. In certain embodiments, theformation is a subterranean shale formation. In one embodiment, themetal complexing agent or reducing agent is introduced during thefracturing process or operation and thus is injected at a pressuregreater than a fracture pressure of the formation. In anotherembodiment, the metal complexing agent or reducing agent is introducedas part of a remediation process and thus is injected at a pressure lessthan a fracture pressure of the formation and introduced after thefracturing process is complete.

In all embodiments of the methods and compositions to control irondisclosed herein the metal complexing agent or reducing agent is atime-released form of the complexing agent or reducing agent. Thetime-release of the metal complexing agent or reducing agent will formsoluble products by interacting with any iron or iron ions within a welland/or the formation, both naturally occurring and introduced iron. Byforming soluble products by interacting with iron, the methods andcompositions will improve the fracturing and/or production of the well.

In certain embodiments, the time-released iron control provides for avarying time-release profile, such that an amount of the complexing orreducing agents is released at a first (earlier) time and another amountof the complexing or reducing agents is released at a second (later)time. In these embodiments of varying time-release profile, there can betwo planned timed releases (2 peaks of release), or multiple plannedtimed releases (three or four peaks of release). In these embodiments oftimed release, preferably there are 2 or 3 planned timed releases (2 or3 peaks of release, See FIGS. 3 and 4 , respectively) and in morepreferred embodiments, there are 2 planned timed releases (2 peaks ofrelease, See FIG. 3 ). In these embodiments of varying time-releaseprofile, the first planned timed release can be as soon as about 10minutes after injection and the second or last planned time-release canbe as late as about 1 month after injection.

In other embodiments, the varying timed release can be a continuoustimed release profile (in the shape of a curve) beginning as early as atabout 10 minutes after injection and ending as long as about 1 monthafter injection (See FIG. 5 ).

The complexing agent or reducing agent as utilized herein is in atime-released form and forms a soluble product by interacting with ironor iron ions. As such, the complexing agent or reducing agent can be anychemical that forms a soluble product. The time-released form may be anyform or delivery system that prevents loss of the agent prior todelivery within the well or the formation where the iron control isneeded and at the desired time(s).

The extent of solubility can be viewed as ranging from infinitely/verysoluble or miscible (such as ethanol in water) to insoluble. The U.S.Pharmacopoeia defines solubility terms as follows and the ranges areadopted herein.

Term Mass parts of solvent required to dissolve 1 mass part of soluteVery soluble <1 Freely soluble 1 to 10 Soluble 10 to 30 Sparinglysoluble 30 to 100 Slightly soluble 100 to 1000 Very slightly soluble1000 to 10,000 Insoluble ≥ 10,000

The solvent as herein is the fluid within the formation or wellbore inwhich the iron product forms and the iron product is the solute.

The term “soluble” as used herein with regard to creating a “soluble”product with iron includes soluble, freely soluble and very soluble, asdefined by the U.S. Pharmacopoeia as set forth above. Thus, the term“soluble” as used herein with regard to creating a “soluble” productwith iron includes the mass parts of solvent required to dissolve 1 masspart of solute (i.e., the iron product) as defined for soluble, freelysoluble and very soluble according to the U.S. Pharmacopoeia. As such,as defined herein, “soluble” means less than (<) 30 mass parts solventto dissolve 1 mass part of solute.

Since the complexing agent or reducing agent will be used in a downholeenvironment and into the formation, it is preferred that the complexingagent or reducing agent and the time-release mechanism not bedetrimental to the environment. The complexing agent may be a chelatingcomplexing agent or a non-chelating complexing agent. The complexingagent as utilized in the treatment fluid may be a single chemical or maybe a mixture of different chemicals functioning as complexing agents.

The reducing agent as used herein is a chemical species that reducesFe³⁺ to Fe²⁺ creating a soluble iron product. The reducing agent asutilized in the treatment fluid may be a single chemical or may be amixture of different chemicals functioning as reducing agents.

The complexing agent can be a metal chelating agent or a non-chelatingcomplexing agent.

In embodiments where the complexing agent is a metal chelating agent, itcan be selected from the group consisting of citric acid, glycine, NTA(nitrilotriacetic acid), EDTA (ethylenediaminetetraacetic acid), PDTA(1,3-propylenediaminetetraacetic acid), HEDTA(hydroxyethylethylenediaminetriacetic acid), DTPA(diethylenetriaminepentaacetic acid), HEIDA (hydroxyethyliminodiaceticacid), CDTA (cyclohexanediaminetetraacetic acid), DPAS(diphenylaminesulfonic acid), EDDHA(ethylenediaminedi(o-hydroxyphenylacetic) acid), HACA(hydroxyaminopolycarboxylic acid), oxalic acid, malonic acid, succinicacid, malic acid, maleic acid, tartronic acid, tartaric acid, glutaricacid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacicacid, phthalic acid, terephthalic acid, aconitic acid, carballylic acid,trimesic acid, isocitric acid, phosphonic acids, and mixtures thereof.In certain embodiments, the metal chelating agent is selected from thegroup consisting of citric acid, glycine, NTA (nitrilotriacetic acid),EDTA (ethylenediaminetetraacetic acid), oxalic acid, malonic acid,succinic acid, phosphonic acids, and mixtures thereof.

In embodiments where the complexing agent is a non-chelating complexingagent, it can be selected from the group consisting of formic acid,acetic acid, propionic acid, lactic acid, gluconic acid, glucoheptonicacid, and mixtures thereof. In certain embodiments, the non-chelatingcomplexing agent is formic acid, acetic, acid, or mixtures thereof.

The complexing agent can also be a mixture of a chelating agent and anon-chelating agent. If a mixture of a chelating agent and non-chelatingagent are utilized, the chelating agent can be any one or more of thechelating agents as disclosed herein in combination with any one or moreof the non-chelating agents as disclosed herein.

In embodiments where the agent is a reducing agent, it can be selectedfrom the group consisting of erythorbic acid, ascorbic acid,hydroxylamine, and mixtures thereof.

In embodiments of the methods and time-release compositions, thecomplexing agent or reducing agent may be present in the treatment fluidin an amount of about 0.01 pound per thousand gallons to about 50 poundsper thousand gallons. In certain embodiments, the complexing agent orreducing agent may be present in the treatment fluid in an amount ofabout 0.05 pound per thousand gallons to about 25 pounds per thousandgallons. In other embodiments, the complexing agent or reducing agentmay be present in the treatment fluid in an amount of about 0.1 poundper thousand gallons to about 20 pounds per thousand gallons. Inspecific embodiments, the complexing agent or reducing agent may bepresent in the treatment fluid in an amount of about 1 pound perthousand gallons to about 10 pounds per thousand gallons.

Treatment fluids as disclosed herein comprise a time-released form ofthe complexing agent or reducing agent. The treatment fluids alsocontain water and optionally mutual solvent. Mutual solvents are solublein oil, water and acid-based mixtures. When the treatment fluidcontaining the time-released form of the complexing agent or reducing isused during a fracturing process, the treatment fluid can furtherinclude any additives commonly utilized during the fracturing process.When the treatment fluid containing the time-released form of thecomplexing agent or reducing agent is used during a remediationoperation, the treatment fluid can further include any additivescommonly used in remediation processes.

The additives used in fracturing and/or remediation include, forexample, acids (such as HCl), corrosion inhibitors, friction reducers,biocides, surfactants, and the like. The amount of additives appropriatereadily can be selected by one of ordinary skill in the art and iswithin known ranges. For example, the treatment fluid can furthercontain water, between 1 and 10,000 parts per million (ppm) of acorrosion inhibitor, between 1 and 10,000 ppm of a biocide, and mutualsolvent. The treatment fluid can further contain hydrochloric acid fromtrace amounts to about 30% by weight.

The amount of complexing agent or reducing agent in the treatment fluidcan be varied as needed as the treatment fluid is injected. For example,it may be advantageous to use a higher initial concentration anddecrease the concentration as the injection is performed. In otherembodiments, it may be advantageous to increase the concentration as theinjection is performed.

In certain of these embodiments, the complexing agent is a metalchelating agent and the metal chelating agent is present in thetreatment fluid in an amount of about 0.01 pound per thousand gallons toabout 50 pounds per thousand gallons. In certain embodiments, the metalchelating agent is present in the treatment fluid in an amount of about0.05 pound per thousand gallons to about 25 pounds per thousand gallons.In other embodiments, the metal chelating agent is present in thetreatment fluid in an amount of about 0.1 pound per thousand gallons toabout 20 pounds per thousand gallons. In specific embodiments, the metalchelating agent is present in the treatment fluid in an amount of about1 pound per thousand gallons to about 10 pounds per thousand gallons.

In certain of these embodiments, the agent is a reducing agent and thereducing agent is present in the treatment fluid in an amount of about0.01 pound per thousand gallons to about 50 pounds per thousand gallons.In certain embodiments, the reducing agent is present in the treatmentfluid in an amount of about 0.05 pound per thousand gallons to about 25pounds per thousand gallons. In other embodiments, the reducing agent ispresent in the treatment fluid in an amount of about 0.1 pound perthousand gallons to about 20 pounds per thousand gallons. In specificembodiments, the reducing agent is present in the treatment fluid in anamount of about 1 pound per thousand gallons to about 10 pounds perthousand gallons.

In the compositions and methods as disclosed herein, the complexingagent or reducing agent is provided in a form, type or delivery systemthat prevents loss of the agent prior to delivery at the site within thewell or formation to complex with iron (i.e., prevents loss during theinjection process) and at the desired time(s). In certain embodiments,in the compositions and methods, the complexing agent or reducing agentis provided in a form, type, or delivery system that provides for avarying time-release profile, such that an amount of the complexing orreducing agents is released at a first (earlier) time and another amountof the complexing or reducing agents is released at a second (later)time. In these embodiments of varying time-release profile, there can betwo planned timed releases (2 peaks of release, See FIG. 3 ), ormultiple planned timed releases (three or four peaks of release, SeeFIG. 4 ). In these embodiments of timed release, preferably there are 2or 3 planned timed releases (2 or 3 peaks of release, See FIGS. 3 and 4respectively) and in more preferred embodiments, there are 2 plannedtimed releases (2 peaks of release, See FIG. 3 ). In these embodimentsof varying time-release profile, the first planned timed release can beas soon as about 10 minutes after injection and the last planned timedrelease can be as late as about 1 month after injection.

In other embodiments, the timed release can be a continuous timedrelease profile (in the shape of a curve, See FIG. 5 ) beginning asearly as at about 10 minutes after injection and ending as long as about1 month after injection.

In certain embodiments to obtain the time-released feature, thecomplexing agent or reducing agent may be encapsulated within a suitablecoating. In embodiments with a coating, the complexing agent or reducingagent may be encapsulated in a coating of one layer, two layers, ormultiple layers (such as three or four layers). In embodiments ofvarying timed release, the types of coatings used on amounts ofcomplexing agent or reducing agent, or coating layers applied to thecomplexing or reducing agents, may be different to provide for thevarying timed release.

When a coating is utilized, any coating may be applied to the agent thatwill prevent release of the agent prior to delivery at the site withinthe well or formation to complex with iron and will prevent release ofthe agent until a desired time(s) after injection. In one embodiment,the agent may have a coating that can be disrupted by pressure changesor mechanical forces. These types of coating may be a non-permeableshell, which releases the agent by crushing or rupture. In anotherembodiment, the coating may be dissolvable in the fluid system andreleases the agent by dissolution. In other embodiments, the coating maybe a permeable shell from which the agent is released by diffusion overtime, based on the diffusion rate through the shell material and thethickness of the shell. In yet other embodiments, the coating may be asemipermeable shell, which releases the agent by opening caused byosmotic swelling. In another embodiment, the coating may be a coatingthat is removed by eroding or dissolving from exposure to theenvironment.

In specific embodiments, the coating may be a polymer. In embodiments,the coating may be a latex selected from polyvinylidene chloride orvinylidene methyl acrylate acrylonitrile copolymer. In otherembodiments, the coating may be a starch or polysaccharide. In furtherembodiments, the coating may be a polymer wax mixture or fatty acid. Inembodiments, the coating may be a dry hydrophobic film forming materialor a dry slowly dissolvable material and particulate silica. The coatingfurther may be a paraffin coating.

The coating may be a porous cross-linked hydrophilic polymer such thatwhen contacted with water it prevents the substantial dissolution of theencapsulated agent for a selected time period. In another embodiment,the complexing agent can be preserved by a coating that can be disruptedby pressure changes or mechanical forces, such as fracture closure.

Several time-release encapsulation materials and techniques for use inhydraulic fracturing are known in the art and any suitable technique maybe used that delivers the agent to site within the formation and/or wellwhere it is to be released to complex with iron in the desiredtime-released manner.

When two or multiple layers of coatings are applied as described, theindividual coating layers may be the same or different. If different,the first coating layer may be any of the herein described coatings andthe second coating layer used with the first coating also may be any ofthe herein described coatings. As such, the combination of the firstcoating with a second coating is not limited. If a coating, whichreleases the agent by pressure changes or mechanical forces, such asfracture closure, is utilized in a layered system, then that coatingshould be the outer most coating (i.e., the coating to be releasedfirst).

Multiple coatings layers may include a first coating layer of a fattyacid (e.g., stearic acid, stearic acid/palmitic acid, lauric acid,myristic acid, palmitic acid, stearic acid, arachidic acid) and a secondcoating layer of fixed alkali hydroxide (i.e., NaOH, KOH, CaOH₂), or asoap of the fatty acid. These types of multiple coating layers form anonpermeable shell that release the agent by dissolution. These multiplecoating layers forming a nonpermeable shell may further be utilized withan outer coating of a nonpermeable shell released by crushing.

As described, in certain embodiments, the coating may be a nonpermeableshell that releases the complexing or reducing agent by crushing ordissolution of coating material. Nonpermeable shell coatings thatrelease the complexing or reducing agent by crushing are useful fortimed release of the agents after fracturing is complete when thepressure inside the formation is released. Examples of coatings thatrelease by crushing include glass, gels, ceramics, plastics (See forexample Nolte, US4506734), polyvinylidene chloride copolymers (See forexample Gulbis, 1992. SPE-19433-PA), and the like. In certainembodiments, coatings of polyvinylidene chloride copolymers that releasethe agent by crushing of the coating material are preferred. If acoating that releases the agent by crushing is utilized in a layeredcoating, then that coating should be the outer most coating (i.e., thecoating to be released first). If a coating that releases the agent bycrushing is utilized in an embodiment with a varying time-releaseprofile of two or multiple planned timed releases, then the agentscoating with that coating would be released at the first plannedrelease.

Coatings that form a nonpermeable shell that release the agent bydissolution of the nonpermeable coating are also used herein. Examplesof coatings that release by dissolution include solidified starch orpolysaccharide matrix with absorbed acid (See for example Seighman,US4713251), hydroxypropylguar or other solid hydratable gelling agents(See for example Burnham, US4202795), polyethylene-vinyl acetatecopolymers (See for example King, US4919209),poly(alkyl-2-cyanoacrylate) (See for example Muir, US6162766), polymericcoating such as ethylcellulose, acrylic latex, cellulose, acrylates,styrene/butadienes, polyvinylidene chlorides, styrene/butadiene,polyvinylidene chloride and mixtures thereof (See for example Nelson,US5658861), polyvinyl alcohol, polyvinyl pyrrolidone, polyethyleneglycols, copolymers of vinyl methyl ether and maleic anhydride. (See forexample Alterman, US3983254), and the like.

In other embodiments, the coating may be a permeable shell that releasesthe agent by diffusion. In these embodiments, the coating exhibitsenough permeability that the coated agent inside slowly diffuses outthrough the permeable shell. Examples of these coatings include parylene(See for example Mirakyan, US20150114648), nylon (See for example Gupta,US5164099), partially hydrolyzed acrylate/silica particles (See forexample Norman, US5373901), and certain polymeric matrices (See forexample McDougall, US4670166). Examples of a polymeric matrix coatingthat is permeable and releases the agent by diffusion include acrylicacid, acrylamide, or acrylonitrile or mixture thereof and minor amountsof at least one difunctional hydrophilic comonomer, such as methylenebis-acrylamide.

In other embodiments, the coating may be semipermeable that releases theagent by permeation or rupture caused by osmotic swelling. Fluid enterssemipermeable coatings and creates an osmotic gradient, releasing theagent. In these embodiments, the coating is permeable to some componentof the environment/fracturing fluid and that penetrates the coatingcausing it to swell or rupture releasing the agent. Examples of thesecoatings include crosslinked elastomer (ionically and covalentlycrosslinked neutralized sulfonated elastomeric polymer, See for exampleManalastas, US5110486), sulfonated EPDM rubber (ethylene propylene dienemonomer rubber), sulfonated butyl rubber, sulfonated EPDM terpolymerwith vinyl pyridine copolymer, sulfonated isoprene/styrene rubber,sulfonated isoprene/butadiene rubber, and sulfonated polystyrene, andvinylidene/methyl acrylate/acrylonitrile (See for example Walles,US4741401).

In further embodiments, the coating may release the encapsulated agentby dissolving or eroding of encapsulating material. These coatings maydissolve or erode by exposure to fluid. Examples of coatings thatrelease by dissolving or eroding include a polymer/wax mixture (See forexample King, US4919209). In the embodiment of a polymer/wax mixture,the polymer may be polyolefin, polyolefin-vinyl acetate copolymer, ormixture thereof. Preferred polyolefin-vinyl acetate copolymers includeethylene-vinyl acetate copolymers. In certain embodiments, usingcoatings of a polymer/wax mixture that release by eroding or dissolvingare preferred.

In the embodiments utilizing a varying time-release profile, such thatan amount of the complexing or reducing agents is released at a first(earlier) time and a second amount of the complexing or reducing agentsis released at a second (later) time, the planned varying time-releaseprofile may be achieved by using different types of coatings (onecoating that will release faster/sooner than a different second coating)or differing layers of coatings (some agents with one layer of coatingand other agents with two or multiple layers of coatings). The plannedvarying time-release profile may be achieved by using both differenttypes of coatings and/or differing layers of coatings. The time-releaseprofile may be planned such that a certain percentage of the agent isdesigned to release as soon as about 10 minutes after injection andanother or remaining percentage of the agent releases as long as aboutone month after injection. The time-release profile may be continuousover the time period, or there may different peaks of release (forexample, two, three, or four peaks of release based on the type ofcoatings, or layers of coating, utilized).

The percentage of agent designed to release at the first time may beabout 25%, about 30%, about 40%, about 50%, about 60%, or about 75% byweight. The percentage of agent designed to have a release later thanthe first release correspondingly may be about 75%, about 70%, about60%, about 50%, about 40%, or about 25% by weight. In certainembodiments the percentage of agent designed to release at a first timeis about 35-65% by weight and the percentage of agent designed torelease at a second later time is about 65-35% by weight. In oneembodiment, the percentage of agent designed to release at a first timeis about 50% by weight and the percentage of agent designed to releaseat a second later time is about 50% by weight.

In the alternative the coated agent may be designed/coated to releasecontinuously from an initial release time to a final release time at asteady release rate. The initial release time may be about 10 minutesafter injection and the final release time may be about 1 month afterinjection.

In certain embodiments the varying time-release profile may be achievedby having two different types of coated complexing agents or reducingagents. In certain embodiments, a first amount of agent may be coatedwith a nonpermeable shell which releases the agent by crushing and asecond amount of agent may be coated with a nonpermeable shell whichreleases the agent by dissolution. For example, a first amount of agentmay be coated with polyvinylidene chloride copolymers and a secondamount of agent may be coated with a polymeric coating that releases bydissolution, such as ethylcellulose, acrylic latex, polyvinyl alcohol,polyvinyl pyrrolidone, polyethylene glycols, or copolymers of vinylmethyl ether and maleic anhydride. In this embodiment, the first amountof agent would release at a first planned timed release.

In other embodiments, a first amount of agent may be coated with anonpermeable shell which releases the agent by crushing and a secondamount of agent may be coated with a permeable shell which releases theagent by diffusion. For example, a first amount of agent may be coatedwith polyvinylidene chloride copolymers and a second amount of agent maybe coated with parylene. In this embodiment, the first amount of agentwould release at a first planned timed release.

In other embodiments, a first amount of agent may be coated with anonpermeable shell which releases the agent by crushing and a secondamount of agent may be coated with an encapsulating material whichreleases by eroding or dissolving. For example, a first amount of agentmay be coated with polyvinylidene chloride copolymers that release bycrushing and a second amount of agent may be coated with a polymer/waxmixture which releases by eroding or dissolving. In this embodiment, thefirst amount of agent would release at a first planned timed release.

In additional embodiments, an amount of agent may be coated with apermeable shell released by diffusion or a nonpermeable shell releasedby dissolution and a second amount of agent may be coated with twolayers - an inner layer of permeable shell released by diffusion and anouter layer of encapsulating material released by eroding or dissolving(such as a polymer/wax mixture).

In other embodiments, the agents may be coated with varying thicknessesof the desired coating(s) providing for a varied time-release, beginningat an initial first release time that is at least about 10 minutes afterinjection and ending at a final release time that is no more than about1 month after injection. For example, a first amount of agent may becoated with a first thickness of a coating that and a second amount ofthe agent may be coated with a coating of the same type that is a secondthickness, which is twice the first thickness of the same coating. Inthese embodiments, the coating may be a permeable shell released bydissolution, a nonpermeable shell released by dissolution, or coatingthat releases by eroding or dissolving. In certain embodiments, if thefirst thickness is planned to dissolve and release the agent about 1week after injection, then the second thickness will be planned todissolve and release the agent in about 2 weeks after injection. Inother embodiments, if the first thickness is planned to dissolve andrelease the agent about 2 weeks after injection, then the secondthickness will be planned to dissolve and release the agent in about amonth after injection. In another embodiment, the agents may be coatedwith thicknesses of coatings that gradually increase in thickness, suchthat the agents release continuously from an initial release time to thefinal release time at a relatively steady release rate.

In other embodiments of time-release not utilizing coatings, thetime-release can be controlled or influenced by differential solubilityor rate of dissolution of the complexing agent or reducing agent in thetreatment fluid as a result of injecting it into the wellbore and/orreservoir. In other embodiments, the time-released form of thecomplexing agent or reducing agent may be adsorbed to a material thatreleases in response to changes in environmental redox potential, pH orion exchange.

In further embodiments not utilizing coatings, the time-released form ofthe complexing agent or reducing agent may be adsorbed to matrix orblended with a matrix and the time-released form of the complexing agentor reducing agent is released from the matrix over time by diffusion.The matrix is selected so that the complexing agent or reducing agenthas sufficient affinity for the matrix. The matrix may be a polymericbead, a ceramic bead, a zeolite, or a polymeric blend. Examples ofmatrices that are known to provide for a timed released include,polymethylmethacrylate (PMMA) (See for example Gupta, US5437331), PMMApolymer blend such as PMMA with PMMA-coethylacrylate-cotrimethylaminoethyl methacrylate chloride, a polymeric beadsuch as polymerized styrene and divinylbenzene (See for example, Won,US4690825). One of skill in the art would select the appropriate matrixdepending on the agent being utilized based on the agent’s affinity forthe matrix.

In other embodiments not utilizing coatings, the release time can becontrolled or influenced by differential solubility or rate ofdissolution of the metal complexing agent in the treatment fluid as aresult of injecting it into the wellbore and/or reservoir. For example,EDTA is slowly soluble at room temperature or low temperatures in whichit could be mixed, and the solubility increases as the temperature ofthe fluid increases in the wellbore or reservoir after injection.

The treatment fluid for time-released iron control comprising atime-released form of a complexing agent or reducing agent, wherein theagent forms a soluble product with iron are utilized in methods forcontrolling iron in a well in a subterranean formation. One embodimentof these methods is graphically illustrated in FIG. 1 (100) and anotherembodiment of these methods is graphically illustrated in FIG. 2 (200).

As illustrated in FIG. 1 , the methods for controlling iron in a well ina subterranean formation as disclosed herein include providing treatmentfluid comprising a time-released form of a complexing agent or areducing agent, wherein the agent forms a soluble product with iron(102). In certain embodiments, the treatment fluid comprises about 0.1pound per thousand gallons to about 20 pounds per thousand gallons ofthe agent. The treatment fluid with complexing agent or reducing agentcan be prepared by methods well known in the art and the agent canreadily be mixed with the treatment fluid.

The treatment fluid is injected into the well of the formation until atleast some of the treatment fluid contacts the formation and iron (104).In one embodiment, the treatment fluid including the complexing orreducing agent is introduced during the fracturing process or operationand thus is injected at a pressure greater than a fracture pressure ofthe formation. In embodiments in which the treatment fluid is introducedduring the fracturing process or operation, at least some amount of thetimed release form of the complexing or reducing agent can be achievedby coating the agent with a nonpermeable shell which releases the agentby crushing. In another embodiment, the treatment fluid including thecomplexing or reducing agent is introduced as part of a remediationprocess and thus is injected at a pressure less than a fracture pressureof the formation.

The treatment fluid including the complexing or reducing agent is shutin the well and maintained in the well in contact with iron, therebyallowing the agent to release over time and then interact with iron toform a soluble product (106 and 108). Thus, although graphicallyillustrated as separate steps, steps 106 and 108 overlap in time.

In certain embodiments, the soluble product is removed from the wellafter the contacting in a mobile fluid phase, but this step is optional.(110).

As illustrated in FIG. 2 , the methods for controlling iron in a well ina subterranean formation as disclosed herein include providing treatmentfluid comprising a time-released form of a complexing agent or areducing agent, wherein the agent releases with a varying time-releaseprofile and forms a soluble product with iron (202). In certainembodiments, the treatment fluid comprises about 0.1 pound per thousandgallons to about 20 pounds per thousand gallons of the agent. Thetreatment fluid with complexing agent or reducing agent can be preparedby methods well known in the art and the agent can readily be mixed withthe treatment fluid and the varying time-release profile may be achievedas described herein.

The treatment fluid is injected into the well of the formation until atleast some of the treatment fluid contacts the formation and iron (204).In one embodiment, the treatment fluid including the complexing orreducing agent is introduced during the fracturing process or operationand thus is injected at a pressure greater than a fracture pressure ofthe formation. In embodiments in which the treatment fluid isintroducing during the fracturing process or operation, at least someamount of the timed release form of the complexing or reducing agent canbe achieved by coating the agent with a nonpermeable shell whichreleases the agent by crushing. In another embodiment, the treatmentfluid including the complexing or reducing agent is introduced as partof a remediation process and thus is injected at a pressure less than afracture pressure of the formation.

The treatment fluid including the complexing or reducing agent is shutin the well and maintained in the well in contact with iron (206, 208,and 210), thereby allowing the agent to release over time and theninteract with iron to form a soluble product. With a varying releaseprofile an amount of agent releases and contacts the formation and ironforming a soluble product with iron at a first time and a second amountof agent releases contacting the formation and iron forming a solubleproduct with iron at a second time (208 and 210). Thus, althoughgraphically illustrated as separate steps, steps 206, 208, and 210 mayoverlap in time.

In certain embodiments, the soluble product may be removed from the wellafter the contacting in a mobile fluid phase, but this step is optional.

In certain embodiments of the method, the time-released iron controlprovides for a varying time-release profile, such that after injectioninto the well, a certain initial amount of the complexing or reducingagents is released at a first (earlier) time and another amount ofcomplexing or reducing agents is released at a second (later) time. Inthese embodiments of varying time-release profile, there can be twoplanned timed releases (2 peaks of release), or multiple planned timedreleases (three or four peaks of release). In these embodiments of timedrelease, preferably there are 2 or 3 planned timed releases (2 or 3peaks of release) and in more preferred embodiments, there are 2 plannedtimed releases (2 peaks of release). In these embodiments of varyingtime-release profile, the first planned timed release can be as soon asabout 10 minutes after injection and the last planned time-release canbe as late as about 1 month after injection.

In other embodiments of the method, the time-released iron controlprovides for a continuous timed release profile (in the shape of acurve) of the agent, beginning as early as at about 10 minutes afterinjection and ending as long as about 1 month after injection.

In certain embodiments, the methods provide for varying time-release ofthe agent. In an embodiment of varying time-release, the method forcontrolling iron in a well in a subterranean formation comprises:providing a treatment fluid comprising a time-released form of acomplexing agent or reducing agent, wherein the time-released complexingagent or reducing agent provides a varying time-release profile andwherein the agent forms a soluble product with iron. The treatment fluidis injected into the well of the formation until at least some of thetreatment fluid contacts the formation and iron and the agent isreleased over time. The treatment fluid is maintained in contact withiron within the formation, thereby allowing the agent to interact withiron over time to form a soluble product. As described herein thevarying timed release can be continuous from an initial release time toa final release time or the varying timed release can be planned timedpeaks of release.

In an embodiment of varying time-release, the methods for controllingiron in a well in a subterranean formation comprise: providing atreatment fluid comprising a first amount of time-released complexingagent or reducing agent and a second amount of time-released complexingagent or reducing agent, wherein the first amount of agent is releasedat a first time and the second amount of agent is released at a secondtime and wherein the agents form a soluble product with iron. Thetreatment fluid is injected into the well of the formation until atleast some of the treatment fluid contacts the formation and iron and isreleased over time. The treatment fluid is maintained in contact withiron within the formation, thereby allowing the agent to interact withiron over time to form a soluble product.

In the methods as described herein, in embodiments the subterraneanformation can be a subterranean shale formation. The iron to becontrolled can be both naturally occurring and introduced iron, andthereby improve performance of the well and/or the fracturing operation.

Unless otherwise indicated, all numbers expressing quantities ofingredients, properties, and so forth used in the specification andclaims are to be understood as being modified in all instances by theterm “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the following specification andattached claims are approximations that may vary depending upon thedesired properties sought to be obtained by the present invention.

As used herein, “about” refers to a degree of deviation based onexperimental error typical for the particular property identified. Thelatitude provided the term “about” will depend on the specific contextand particular property and can be readily discerned by those skilled inthe art. The term “about” may include +/- 10 percent for example and isnot intended to either expand or limit the degree of equivalents whichmay otherwise be afforded a particular value. Further, unless otherwisestated, the term “about” shall expressly include “exactly,” consistentwith the discussions regarding ranges and numerical data.Concentrations, amounts, and other numerical data may be expressed orpresented herein in a range format. It is to be understood that such arange format is used merely for convenience and brevity and thus shouldbe interpreted flexibly to include not only the numerical valuesexplicitly recited as the limits of the range, but also to include allthe individual numerical values or sub-ranges encompassed within thatrange as if each numerical value and sub-range is explicitly recited. Asan illustration, a numerical range of “about 4 percent to about 7percent” should be interpreted to include not only the explicitlyrecited values of about 4 percent to about 7 percent, but also includeindividual values and sub-ranges within the indicated range. Thus,included in this numerical range are individual values such as 4.5, 5.25and 6 and sub-ranges such as from 4-5, from 5-7, and from 5.5-6.5; etc.This same principle applies to ranges reciting only one numerical value.Furthermore, such an interpretation should apply regardless of thebreadth of the range or the characteristics being described.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contain certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements.

It will be clear that the methods described herein are well adapted toattain the ends and advantages mentioned as well as those inherenttherein. Those skilled in the art will recognize that the methods withinthis specification may be implemented in many manners and as such is notto be limited by the foregoing exemplified embodiments and examples.While various embodiments have been described for purposes of thisdisclosure, various changes and modifications may be made which are wellwithin the scope of the present invention. Numerous other changes may bemade which will readily suggest themselves to those skilled in the artand which are encompassed in the spirit of the disclosure.

1-24. (canceled)
 25. A treatment fluid for time-released iron control ina well in a subterranean formation comprising water and a time-releasedform of a complexing agent consisting of oxalic acid, malonic acid,succinic acid, phosphonic acids, adipic acid, or mixtures thereofencapsulated within a coating, wherein the time-released form of theagent is present in the treatment fluid in an amount of about 0.01 poundper thousand gallons of the fluid to about 50 pounds per thousandgallons of the fluid and the agent forms a soluble product with iron.26. The fluid of claim 25, wherein the time-released form of thecomplexing agent is in a form that provides a varying time-releaseprofile for the agent.
 27. The fluid of claim 26, comprising a firstamount of the time-released agent in a form to release at a first timeand a second amount of the time-released agent in a form to release at asecond time.
 28. The fluid of claim 26, wherein the time-releasedcomplexing agent provides a continuous timed release profile from astart time to an end time.
 29. The fluid of claim 25, wherein thecoating is a non-permeable shell that releases the agent by crushing ordissolution of the coating.
 30. The fluid of claim 29, wherein thecoating is polyvinylidene chloride copolymers.
 31. The fluid of claim25, wherein the coating is a nonpermeable shell that releases the agentby dissolution.
 32. The fluid of claim 25, wherein the coating is apermeable shell that releases the agent by diffusion.
 33. The fluid ofclaim 25, wherein the coating is a semipermeable shell that releases theagent by permeation or rupture caused by osmotic swelling.
 34. The fluidof claim 25, wherein the coating has multiple layers.
 35. A treatmentfluid for time-released iron control in a well in a subterraneanformation comprising water and a time-released form of two or morecomplexing agents selected from the group consisting of citric acid,formic acid, acetic acid, propionic acid, lactic acid, gluconic acid,glucoheptonic acid, oxalic acid, malonic acid, succinic acid, adipicacid, and phosphonic acids encapsulated within a coating, wherein thetime-released form of the agents are present in the treatment fluid inan amount of about 0.01 pound per thousand gallons of the fluid to about50 pounds per thousand gallons of the fluid and the agents form asoluble product with iron.
 36. The fluid of claim 35, wherein thecomplexing agents are citric acid and formic acid, acetic acid,propionic acid, lactic acid, gluconic acid, glucoheptonic acid, oxalicacid, malonic acid, succinic acid, adipic acid, or phosphonic acids. 37.The fluid of claim 35, wherein the time-released form of the complexingagents is in a form that provides a varying time-release profile for theagents.
 38. The fluid of claim 35, comprising a first amount of thetime-released agents in a form to release at a first time and a secondamount of the time-released agents in a form to release at a secondtime.
 39. The fluid of claim 35, wherein the time-released complexingagents provides a continuous timed release profile from a start time toan end time.
 40. The fluid of claim 35, wherein the coating is anon-permeable shell that releases the agent by crushing or dissolutionof the coating.
 41. The fluid of claim 35, wherein the coating is apermeable shell that releases the agent by diffusion.
 42. The fluid ofclaim 35, wherein the coating is a semipermeable shell that releases theagent by permeation or rupture caused by osmotic swelling.
 43. The fluidof claim 35, wherein the coating has multiple layers.
 44. A method forcontrolling iron in a well in a subterranean formation comprising:providing a treatment fluid comprising water and a time-released form oftwo or more complexing agents selected from the group consisting ofcitric acid, formic acid, acetic acid, propionic acid, lactic acid,gluconic acid, glucoheptonic acid, oxalic acid, malonic acid, succinicacid, adipic acid, and phosphonic acids encapsulated within a coating,wherein the time-released form of the complexing agents are present inthe treatment fluid in an amount of about 0.01 pound per thousandgallons of the fluid to about 50 pounds per thousand gallons of thefluid and the agents form a soluble product with iron; injecting thetreatment fluid into the well of the formation until at least some ofthe treatment fluid contacts the formation and iron, wherein the ironincludes iron originating from well equipment; and maintaining thetreatment fluid in contact with iron within the formation, therebyallowing the agents to release and interact with iron to form a solubleproduct.
 45. The method of claim 44, wherein the complexing agents arecitric acid and formic acid, acetic acid, propionic acid, lactic acid,gluconic acid, glucoheptonic acid, oxalic acid, malonic acid, succinicacid, adipic acid, or phosphonic acids.
 46. The method of claim 44,wherein the subterranean formation is a subterranean shale formation.47. The method of claim 44, wherein the time-released form of thecomplexing agents is in a form that provides a varying release profilefor the agent.
 48. The method of claim 47, wherein the time-releasedform of the complexing agents comprises a first amount of time-releasedagents in a form to release at a first time and a second amount oftime-released agents in a form to release at a second time.
 49. Themethod of claim 44, wherein the iron includes iron originating from pipeerosion.
 50. The method of claim 44, wherein the time-released form ofthe complexing agents is released 10 minutes after injection to 1 monthafter injecting.